# std::compare_three_way

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< cpp‎ | utility

Defined in header `<compare>` |  |
Defined in header `<functional>` |  |
struct compare_three_way; |  | (since C++20)

Function object for performing comparisons. Deduces the parameter types and the return type of the function call operator.

### Nested types

Nested type | Definition
`is_transparent` | unspecified

### Member functions

operator() | obtains the result of three-way comparison on both arguments (public member function)

## std::compare_three_way::operator()

template< class T, class U > constexpr auto operator()( T&& t, U&& u ) const; |  |

Given the expression std::forward<T>(t) <=> std::forward<U>(u) as expr:

- If expr results in a call to built-in operator<=> comparing pointers, given the composite pointer type of t and u as `P`:

  - Compares the two converted pointers (of type `P`) in the implementation-defined strict total order over pointers:
    - If t precedes u, returns `std::strong_ordering::less`.
    - If u precedes t, returns `std::strong_ordering::greater`.
    - Otherwise, returns `std::strong_ordering::equal`.
  - If the conversion sequence from `T` to `P` or the conversion sequence from `U` to `P` is not equality-preserving, the behavior is undefined.

- Otherwise:

  - Returns the result of expr.
  - If std::three_way_comparable_with<T, U> is not modeled, the behavior is undefined.

This overload participates in overload resolution only if std::three_way_comparable_with<T, U> is satisfied.

### Example

Run this code

    #include <compare>
    #include <iostream>

    struct Rational
    {
        int num;
        int den; // > 0

        // Although the comparison X <=> Y will work, a direct call
        // to std::compare_three_way{}(X, Y) requires the operator==
        // be defined, to satisfy the std::three_way_comparable_with.
        constexpr bool operator==(Rational const&) const = default;
    };

    constexpr std::weak_ordering operator<=>(Rational lhs, Rational rhs)
    {
        return lhs.num * rhs.den <=> rhs.num * lhs.den;
    }

    void print(std::weak_ordering value)
    {
        value < 0 ? std::cout << "less\n" :
        value > 0 ? std::cout << "greater\n" :
                    std::cout << "equal\n";
    }

    int main()
    {
        Rational a{6, 5};
        Rational b{8, 7};
        print(a <=> b);
        print(std::compare_three_way{}(a, b));
    }

Output:

    greater
    greater

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3530 | C++20 | syntactic checks were relaxed while comparing pointers | only semantic requirements are relaxed

### See also

ranges::equal_to (C++20) | constrained function object implementing x == y (class)
ranges::not_equal_to (C++20) | constrained function object implementing x != y (class)
ranges::less (C++20) | constrained function object implementing x < y (class)
ranges::greater (C++20) | constrained function object implementing x > y (class)
ranges::less_equal (C++20) | constrained function object implementing x <= y (class)
ranges::greater_equal (C++20) | constrained function object implementing x >= y (class)

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